In conventional gas turbines a combustor is made from a number of individual burners which feed hot gas into a first stage with nozzle guide vanes that are located downstream of the combustor. The guide vanes direct the hot gases from the individual burners and the air from the compressor stage in a predetermined direction. Moreover, the guide vanes comprise nozzles through which cooling air may be exhausted in order to cool the surfaces of the guide vanes.
In a conventional combustor stage of the turbine, a number of individual burners are located circumferentially around the centre of the turbine. Thus, there is some tangential gas temperature variation associated with the flow of the hot gases from the individual combustors in the downstream direction. As the number of individual burners decrease, the amount of tangential gas temperature variation increases because between the burners a lower temperature is generated and close to the burners a higher temperature is generated.
This tangential temperature variation leads to a varying temperature profile at each downstream nozzle guide vane, wherein the temperature profile on each nozzle guide vane is dependent on the position of the nozzle guide vane relative to the individual burners, i.e. relative to the installation location of the nozzle guide vane inside the turbine.
The metal temperature is a critical aspect to the life of each nozzle guide vane. The metal temperature may be controlled by the use of cooling air. However, a use of an excessive amount of cooling air reduces the power and efficiency of the gas turbine. In conventional cooling systems, the amount of cooling air has to be biased to match the gas temperature profile for the nozzle guide vane that is exposed to the hottest temperature so that all nozzle guide vanes have the same acceptable life.
A conventional nozzle guide vane (NGV) comprises a plurality of holes through which a cooling fluid may be exhausted for providing a film cooling on the surfaces of the NGV. The NGV may comprise impingement plates or tubes that are used to meter the air into the correct locations. These impingement plates or tubes are located within the NGV for cooling the inner wall of the NGV.
In a conventional embodiment of the impingement cooling system, the cooling air that streams within each NGV, in particular within the impingement plates or tubes, is for all installed NGVs the same or is controlled by complex biasing valves.
CA 2 596 040 A1 discloses a cooling air distribution system that distributes the cooling air upstream of the leading edge of a guide vane aerofoil. A plurality of openings are installed into a support flange so that cooling air may be injected inside a combustion zone for cooling the leading edge of a guide vane aerofoil from the outside.
EP 1 039 096 A2 discloses a guide vane in which an impingement tube is installed. The impingement plate comprises exhaustion holes that guide cooling air to the inner surface of the guide vane for cooling the inner wall of the guide vane.
EP 1 544 414 B1 discloses a guide vane that comprises an impingement tube with exhaustion holes for guiding cooling air from the inside to the inner wall surface of the guide vane. Some exhaustion holes for the cooling fluid of a guide vane may differ to adjacent exhaustion holes of adjacent guide vanes.
EP 1 319 806 A2 and U.S. Pat. No. 4,785,624 disclose complex adjustment devices such as biasing valves and adjustment systems for adjusting the size of an exhaustion hole.
GB 2 450 405 A discloses a gas turbine nozzle with differently cooled vanes, wherein the differences in cooling may be achieved by varying the configuration of film cooling holes and the thickness of thermal barrier coating.